1. Field of the Invention
The present invention concerns a method to generate MR images of an examination region in a neurological sequence protocol and a magnetic resonance system for implementing such a method.
2. Description of the Prior Art
In magnetic resonance tomography it is known to acquire multiple pulse sequences to clarify neurological questions in an examination subject. After an overview pulse sequence a T1-weighted pulse sequence is typically used in order to create T1-weighted MR images of a desired region of the examination subject. Furthermore, T2-weighted pulse sequences are acquired to generate T2-weighted MR images. Often to clarify neural questions that pertain to exposures of the brain, the MR signals are acquired so as to exhibit tissue intensity of the cerebrospinal fluid (CSF). White brain matter exhibits a T2 time of approximately 80 ms and grey brain matter exhibits a T2 time of 100 ms, and the cerebrospinal fluid exhibits a T2 time of 2000 ms. In the T2-weighted imaging sequences the high signal of cerebrospinal fluid interferes and marks the other signals so that a diagnosis (for example of multiple sclerosis) is difficult. For this reason the known FLAIR pulse sequence (Fluid Attenuation Inversion Recovery) is typically additionally used. With this imaging sequence with a long inversion time, the magnetization is inverted overall, and the actual imaging is started at a point in time when the longitudinal magnetization of the interfering tissue component is zero. Due to the very long T1 time of the cerebrospinal fluid of approximately 3000 ms, the total acquisition time for this pulse sequence is very long. The examination subject thus must remain for a very long time in the magnetic resonance system. Furthermore, not all questions can be satisfactorily resolved with the FLAIR technique.
From DE 19 616 387 A1 a pulse sequence for a magnetic resonance system is known in which, after an excitation in two time spans at different intervals for excitation, 2 groups of magnetic resonance signals are acquired, with the second group of MR signals being acquired in a time period in which the tissue with a significantly longer T2 time (for example the cerebrospinal fluid) delivers the significant signal contribution. An image is subsequently reconstructed based on the difference of the MR signals of the first and second groups.
The multiple contrast pulse sequence is also known under the name HIRE (High Intensity Reduction Sequence). The interfering high signal of the cerebrospinal fluid is removed via subtraction of the signals of the second group (that were acquired at an echo time of 200 ms, for example) from the MR signals of the first group (with an echo time of 90 ms, for example). However, through this subtraction tissue intensities are also reduced that have T2 values between the values of grey brain matter and white brain matter and the value of the CSF as, for example, is the case for an edema of a tumor or the regions of multiple sclerosis. Moreover, the signal-to-noise ratio is impaired by the subtraction since the noise level is hereby increased by a factor of √2. From DE 101 218 021 it is known to avoid these disadvantages in that the MR signals of the second group are multiplied with a weighting factor, wherein the weighting factor depends on the absolute value [amplitude; magnitude] of the respective signal values of the second group.